1. Field of the Invention
Embodiments of the present invention relate generally to a method for obtaining a cathode and an electron beam writing apparatus, and more specifically, for example, relate to a method for selecting a cathode of a beam source used in an electron beam writing apparatus.
2. Description of Related Art
In recent years, with high integration of LSI, the line width (critical dimension) required for circuits of semiconductor devices is becoming progressively narrower. As a method for forming an exposure mask (also called a reticle) used to form circuit patterns on these semiconductor devices, the electron beam (EB) writing technique having excellent resolution is employed.
FIG. 12 is a conceptual diagram explaining operations of a variable shaped electron beam (EB) writing or “drawing” apparatus. The variable shaped electron beam writing apparatus operates as described below. A first aperture plate 410 has a quadrangular aperture 411 for shaping an electron beam 330. A second aperture plate 420 has a variable shape aperture 421 for shaping the electron beam 330 having passed through the aperture 411 of the first aperture plate 410 into a desired quadrangular shape. The electron beam 330 emitted from a charged particle source 430 and having passed through the aperture 411 is deflected by a deflector to pass through a part of the variable shape aperture 421 of the second aperture plate 420, and thereby to irradiate a target object or “sample” 340 placed on a stage which continuously moves in one predetermined direction (e.g., the x direction) during writing. In other words, a quadrangular shape that can pass through both the aperture 411 of the first aperture plate 410 and the variable shape aperture 421 of the second aperture plate 420 is used for pattern writing in a writing region of the target object 340 on the stage continuously moving in the x direction. This method of forming a given shape by letting beams pass through both the aperture 411 of the first aperture plate 410 and the variable shape aperture 421 of the second aperture plate 420 is referred to as a variable shaped beam (VSB) system.
In electron beam writing, an electron gun assembly is used. As a cathode material of the electron gun assembly, lanthanum hexaboride (LaB6), cerium hexaboride (CeB6), hafnium carbide (HfC), or the like which is in sintered or crystalline form is used. Such cathode material is used as electron sources or emitters in various electronic beam apparatuses (e.g., lithographic apparatuses, scanning electron microscopes (SEMs), transmission electron microscopes (TEMs), etc.). The cathode is formed to be a tapered shape with a cone angle, or a conical shape with a truncated flat tip (top), for example.
FIG. 13 shows an example of a cathode whose conical surface is coated with carbon. As shown in FIG. 13, the conical surface made of the cathode material is coated with carbon. By this structure, the electron emission surface is limited to the cathode upper surface, and therefore, the emission area can be limited. By utilizing this, to improve the brightness (luminance or intensity) of the electron gun has been attempted.
FIG. 14 shows an example of the upper surface of a cathode whose side surface is coated with carbon. As shown in FIG. 14, the electron emission surface of the cathode has been corroded by a chemical interaction between the carbon coating and the cathode material such as LaB6 or CeB6. Moreover, as shown in FIG. 14, the periphery of the electron emission surface in contact with the carbon coating appears to have been damaged (for example, pitted and/or etched). In fact, the electron emission surface area is impaired, and thus, is no longer capable of efficiently emitting electrons. Then, a structure has been proposed in which a gap (space) is provided between the cathode material and the carbon coating, in the vicinity of the periphery of the electron emission surface of a cathode (refer to, e.g., Japanese Unexamined Patent Publication No. 2012-069364). Thereby, it is possible to inhibit the chemical reaction between the carbon coating and the electron emission surface periphery of the cathode.
Here, there is a problem in that if the gap between the cathode body (emitter) and the carbon coating is large, the brightness is deteriorated. Since variation occurs in products due to a process of applying a sacrificial film for forming the gap and of removing it, dispersion occurs in gap distance of produced products (cathodes). Cathodes with large gaps are unable to realize high brightness required for the electron beam writing apparatus. It is conventionally difficult to discern whether the produced cathode can achieve high brightness or not from its appearance. Therefore, there has occurred a problem in that since brightness deficiency is found after actually installing a produced cathode in a writing apparatus, the cathode turns out to be unusable, which is a reason to lower the apparatus reliability.